In the design of internal combustion engine components, especially for diesel engines, there is a requirement to increase power density, higher compression ratios, increased fuel injection pressures, exhaust gas recirculation and after-treatment of exhaust gases. Each of these places high mechanical and thermal loads on the pistons of the engines. For example, a steel piston in a diesel engine will be exposed to more than 200 bar of pressure and temperatures in the range of about 600° C. in the combustion chamber, accompanied by high temperature gradients and high inertial forces due to high reciprocating speeds. These demands are accompanied by a requirement to not increase, but, indeed, to reduce the mass of the piston.
One feature of the piston structure known in the art, as illustrated, for example, by U.S. Pat. No. 6,491,013, to Gaiser and assigned to Federal Mogul, is a gallery beneath the top wall of the piston body and radially inward from the ring belt. While many references, including Gaiser '013, refer to this gallery as being “closed,” the need to allow circulation of cooling fluid mandates that there are openings to let the fluid in and out of the gallery, so it is not actually closed, but that terminology will be used here, as it is in the prior art. As described by Gaiser '013, it is conventional to form the piston body with the closed gallery from a top part and a bottom part, which are then joined together by a technique such as friction welding. The respective parts are preferably investment castings, but they may be forged from a casting as a monobloc structure with an internal recess for receiving a connecting rod. The piston will also conventionally have a bowl crown formed at the top of the piston body to reduce thermal loads and lighten the overall mass.
In recent years, a trend has been to move to aluminum pistons for use in gasoline internal combustion engines for automobiles, but steel is still clearly preferred for diesel pistons, as it has higher strength at higher operating temperatures. For definitional purposes, this specification will refer to the pistons as being used for heavy-duty engines (or HDEs), which will be further classified as either “medium HDEs” or “heavy HDEs.” The distinction, as set by the US Environmental Protection Agency, is that HDEs have a power range of 200 to greater than 3500 horsepower, but that medium HDEs are in the weight range of 19501 to 33000 pounds, while heavy HDEs are heavier than 33000 pounds.
To properly form the cooling gallery, the manufacturing sequence used includes at least forging, machining, welding and heat treatment. An exemplary embodiment of such techniques is provided by U.S. Pat. No. 8,286,852 to Kolbe and assigned to Gesenkschmiede Schneider GmbH. In that patent, a head member and a skirt member are separately forged. Each member provides an open portion of a cooling gallery. By setting up a pair of unsymmetrical contact surfaces on the respective parts, a welding bead formed by friction welding is located predominantly outside of the cooling gallery produced by the friction welding.
An even more-recent patent, U.S. Pat. No. 9,216,474, to Wandrie and assigned to Industrial Parts Depot, Torrance, Calif., continues to use a friction welding technique of a pair of separately formed parts to provide the closed gallery.
It is therefore an unmet advantage of the prior art to provide a new and non-obvious method of combining technologies in a hybrid manner to manufacture heavy duty steel pistons with a cooling gallery.